cmd/6g, cmd/8g: disable Duff's device on NaCl.

Native Client forbids jumps/calls to arbitrary locations and
enforces a particular alignement, which makes the Duff's device
ineffective.

LGTM=khr
R=rsc, dave, khr
CC=golang-codereviews
https://golang.org/cl/84400043

src/cmd/6g/cgen.c

@@ -1448,7 +1448,7 @@ sgen(Node *n, Node *ns, int64 w)
 		gins(ACLD, N, N);
 	} else {
 		// normal direction
-		if(q > 128) {
+		if(q > 128 || (nacl && q >= 4)) {
 			gconreg(movptr, q, D_CX);
 			gins(AREP, N, N);	// repeat
 			gins(AMOVSQ, N, N);	// MOVQ *(SI)+,*(DI)+

src/cmd/6g/ggen.c

@@ -77,7 +77,7 @@ zerorange(Prog *p, vlong frame, vlong lo, vlong hi, uint32 *ax)
 		for(i = 0; i < cnt; i += widthreg) {
 			p = appendpp(p, AMOVQ, D_AX, 0, D_SP+D_INDIR, frame+lo+i);
 		}
-	} else if(cnt <= 128*widthreg) {
+	} else if(!nacl && (cnt <= 128*widthreg)) {
 		p = appendpp(p, leaptr, D_SP+D_INDIR, frame+lo, D_DI, 0);
 		p = appendpp(p, ADUFFZERO, D_NONE, 0, D_ADDR, 2*(128-cnt/widthreg));
 		p->to.sym = linksym(pkglookup("duffzero", runtimepkg));
@@ -1119,7 +1119,7 @@ clearfat(Node *nl)
 	savex(D_AX, &ax, &oldax, N, types[tptr]);
 	gconreg(AMOVL, 0, D_AX);
 
-	if(q > 128) {
+	if(q > 128 || (q >= 4 && nacl)) {
 		gconreg(movptr, q, D_CX);
 		gins(AREP, N, N);	// repeat
 		gins(ASTOSQ, N, N);	// STOQ AL,*(DI)+

src/cmd/8g/cgen.c

@@ -1315,7 +1315,7 @@ sgen(Node *n, Node *res, int64 w)
 	} else {
 		gins(ACLD, N, N);	// paranoia.  TODO(rsc): remove?
 		// normal direction
-		if(q > 128) {
+		if(q > 128 || (q >= 4 && nacl)) {
 			gconreg(AMOVL, q, D_CX);
 			gins(AREP, N, N);	// repeat
 			gins(AMOVSL, N, N);	// MOVL *(SI)+,*(DI)+

src/cmd/8g/ggen.c

@@ -75,7 +75,7 @@ zerorange(Prog *p, vlong frame, vlong lo, vlong hi, uint32 *ax)
 		for(i = 0; i < cnt; i += widthreg) {
 			p = appendpp(p, AMOVL, D_AX, 0, D_SP+D_INDIR, frame+lo+i);
 		}
-	} else if(cnt <= 128*widthreg) {
+	} else if(!nacl && cnt <= 128*widthreg) {
 		p = appendpp(p, ALEAL, D_SP+D_INDIR, frame+lo, D_DI, 0);
 		p = appendpp(p, ADUFFZERO, D_NONE, 0, D_ADDR, 1*(128-cnt/widthreg));
 		p->to.sym = linksym(pkglookup("duffzero", runtimepkg));
@@ -176,7 +176,7 @@ clearfat(Node *nl)
 	agen(nl, &n1);
 	gconreg(AMOVL, 0, D_AX);
 
-	if(q > 128) {
+	if(q > 128 || (q >= 4 && nacl)) {
 		gconreg(AMOVL, q, D_CX);
 		gins(AREP, N, N);	// repeat
 		gins(ASTOSL, N, N);	// STOL AL,*(DI)+